1. Field of the Invention
This invention relates generally to heat tolerant and wear resistant coatings for internal combustion engines, and more particularly to a combination of a thermal barrier layer and a wear-resistant layer providing an improved coating for the cylinder walls of internal combustion engines, where the wear coating is self-lubricating to reduce wear on the coated engine parts, and can withstand high operating temperatures.
2. Brief Description of the Prior Art
Inadequate cooling of an engine is a limiting factor as to how much power can be consistently produced by the engine. Therefore, the amount of heat conductively transferred from the engine's working chamber (piston/cylinder or rotor/rotor housing) to the engine's cooling system is preferably kept to a minimum so that the cooling system can adequately remove the conducted heat and maintain the engine at an acceptable operating temperature. Low heat rejection engines have used ceramic insulating coatings to contain the combustion energy within the working chamber of the engine. Many different types of ceramics have been experimented with in an effort to get reasonable wear characteristics in addition to the thermal protection provided by the insulating coating. Recent developments in the adiabatic engine, such as those disclosed in S.A.E. publication SP-738 dated 1988, illustrate the use of ceramics as thermal insulators for engine parts. Although these ceramics are good insulators, they are not good wear surfaces and the roughness of the ceramic surface is ground down by friction rather quickly.
Although ceramic thermal barrier coatings have been very successful at reducing heat transfer through the engine housing, in doing so they have raised the temperature of the internal engine surfaces above the capability of any lubricant to maintain a lubricating film. Recently, some high temperature, self-lubricating materials have been developed for applications where the temperature exceeds the capability of known lubricants. One example is NASA PS-200 self-lubricating composite coating developed by NASA and consisting of a nickel alloy-bonded chromium carbide (or silicon carbide) matrix with dispersed particles of silver and calcium fluoride-barium fluoride eutectic. The silver and fluorides form low shear strength films on sliding surfaces. The silver provides lubrication up to 500.degree. C. and the fluoride eutectic, which undergoes a brittle to ductile transition at temperatures above 500.degree. C., provides lubrication from 500.degree. C. to 900.degree. C.
There is a complex problem, however, with finding a thermal barrier coating and a self-lubricating wear coating that are compatible with one another and can be applied to the working parts of an engine economically and practically.